The invention relates to the manufacture of cutting structures for rotary drill bits for use in drilling or coring holes in subsurface formations.
In particular, the invention is applicable to cutting structures for rotary drill bits of the kind comprising a bit body having a shank for connection to the drill string and an inner passage for supplying drilling fluid to the face of the bit, the bit body carrying a plurality of cutting structures. Each cutting structure comprises a preform cutting element, often in the form of a circular disc, having a front cutting face formed of polycrystalline diamond or other superhard material and a rear face bonded to a carrier of hard, but not superhard, material such as cemented tungsten carbide, the carrier being in turn mounted on the bit body.
Usually, but not essentially, each preform cutting element is a polycrystalline diamond compact comprising two layers: a hard facing layer formed of the polycrystalline diamond, and a backing layer formed of hard, but not superhard, material, usually cemented tungsten carbide, the two layers being bonded together during formation of the cutting element in a high pressure, high temperature forming press.
In one common form of drill bit of the above mentioned type, the carrier comprises a stud or post to which the preform is bonded, the stud or post being received and secured within a socket in the bit body.
In one common method of making such a bit body, the body, or the outer portion thereof, is formed from an infiltrated tungsten carbide matrix by a powder metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered material, such as tungsten carbide, which is then infiltrated with a metal binder alloy, such as a copper alloy, in a furnace so as to form a hard matrix. Formers are normally mounted on the interior surface of the mould so as to define in the finished bit body the aforementioned sockets to receive the studs of the cutting structures. In matrix-bodied bits the studs are usually brazed into their sockets.
In another common form of drill bit, the bit body is machined from steel and the sockets are machined in the bit body. In this case the studs of the cutting structures are often shrink-fitted into the sockets, but they may also be brazed in the sockets.
Conventional two-layer preform cutting elements of the kind referred to above are only thermally stable up to a temperature of about 700.degree. to 750.degree. C. Due to this limitation, problems have arisen in bonding the preforms sufficiently securely to the stud or post. The preforms are normally bonded to the stud or post by brazing and, generally speaking, the strength of a brazed joint depends on the liquidus temperature of the braze alloy--the higher the liquidus temperature the greater the strength. Accordingly, if the brazing is carried out at a temperature which the preform can withstand, the resultant brazed joint may not be sufficiently strong to resist the substantially mechanical forces to which it is subjected during drilling. The problem may also be worsened due to the bond suffering from a significant loss of strength, for example through cracking, when it is reheated for brazing the post or stud into the drill bit body. The joint may also fail as a result of high temperatures reached during drilling.
In order to enable higher temperature brazing processes to be employed, sophisticated cooling techniques have been developed to protect the two-layer preform from the high temperature at which brazing takes place. Such techniques are described for example in U.S. Pat. Nos. 4,225,322, 4,319,707 and 4,527,998. One such technique is sometimes referred to as "LS Bonding".
The last-mentioned patents refer to various high temperature braze alloys which may be used with the bonding methods described. A commonly used alloy is a copper-based brazing alloy sold under the trade name "COCUMAN". Such known brazing alloys, however, can still suffer from the problems mentioned above, in spite of the use of the LS bonding method, and in particular the loss of strength when re-heated for brazing the cutting structure into the drill bit body. For example, when "COCUMAN" is used as a brazing alloy the bond may initially have a strength of the order of 60.times.10.sup.3 psi. However, after the cutting structure has been braze fitted to the bit body the strength may have fallen to something of the order of 40.times.10.sup.3 psi, which is inadequate. It is desirable that the bond strength should be a minimum of 60.times.10.sup.3 psi after the cutting structure has been braze fitted to the bit body.
It might be thought advantageous to have bond strengths well in excess of 60.times.10.sup.3 psi, but this is not as obvious as it might at first appear. The mechanisms employed to strengthen materials are based on limiting dislocation movement and, as a result, deformation and toughness are also limited. Very strong, but brittle, alloys could thus fail in service under impact conditions instead of in the ductile failure mode which currently prevails. Hence, toughness is a second important requirement for any new bonding alloy for bonding cutters to their carriers.
Various bonding alloys providing higher strengths than "COCUMAN" have been developed but without notable success. For example, European Patent Specification No. 0,213,300 describes brazing alloys containing boron. These alloys have a claimed strength of the order of 100.times.10.sup.3 psi, i.e. nearly twice the strength of "COCUMAN". However, bonded structures using such alloys have still been found to fail in service. Although the mechanism has not been established with certainty, it is believed that the boron within the braze alloy reacts with the cemented carbide carrier to which the cutter is brazed, thereby reducing the strength and toughness of the bonded assembly, which may lead to failure in service.
The present invention sets out to provide an improved method of manufacturing cutting structures using an improved bonding alloy.